The present invention relates to an optical depolarizer which converts a polarized beam to an unpolarized beam.
A conventional optical fiber cable which is used for an optical communication has the nature that the polarization of the beam in the cable depends upon the temperature change of the cable, and/or the external stress to the cable. Therefore, in an optical fiber communication system which includes an optical element which depends upon polarization, the detected level is affected by the polarization, and further, the polarization affects to an error rate of the communication system.
In order to solve the above problem, an input optical beam is de-polarized before the beam is applied to the optical element with polarization dependency so that the output level of the optical beam of the polarization dependent element becomes constant. The de-polarizer or de-polarization in the present specification means an element for converting a polarized beam to a non-polarized beam.
A prior de-polarizer is shown in FIG. 1, in which a pair of crystals in the shape of triangular column having complex refractive index are coupled together so that the angle between the polarization axes of two crystals is 90.degree. or 45.degree.. The operational principle of that device is shown in FIG. 2. In FIG. 2, an input optical beam 2-1 which expands spacially is applied to the de-polarizer 2-2, then, each optical energy at each portion is affected by the particular phase difference. Therefore, the output beam of the de-polarizer 2-2 is the sum of optical waves with different polarization, and the output beam 2-3 is as a whole non-polarized beam which has uniform output in all the polarization direction.
However, the de-polarizer in FIG. 1 has the disadvantages that the size of the device is large, and there is some difficulty to couple the device with an optical fiber since the structure of the crystal is not an optical waveguide.